System and method for sequestering emissions from engines

ABSTRACT

A system is provided for sequestering carbon dioxide emitted in exhaust gas from an engine. The exhaust gas may be cooled using heat exchange techniques, for example reverse flow heat exchangers. Carbon dioxide may be separated from the exhaust gases, for example using fractional distillation, heat exchangers, and turbo chargers. The separated carbon dioxide may be stored, for example using injection deep into the earth for enhancing natural gas and oil extraction. Alternatively, the separated carbon dioxide may be cooled to solid carbon dioxide and stored, for example, in an LNG tank for use as ballast onboard a ship or for use in industry.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priority and benefit of U.S. patent application No. 61/293,609 titled “Zero-Emissions Engines,” filed Jan. 8, 2010. The disclosures of the above U.S. patent application is incorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

The present disclosure generally relates to emission control systems. The present disclosure more specifically relates to sequestering emissions from an engine during operation.

2. Description of Related Art

The major components of engine exhaust are nitrogen, water vapor, and carbon dioxide. For example, 100 grams (g) of exhaust gas may include approximately 73 g of nitrogen gas, 12 g of water vapor, 14 g of carbon gas dioxide, and 1 g of trace pollutants. Water vapor can be condensed from exhaust gas according to standard techniques and safely returned to the environment. Trace pollutants can be removed using emissions control systems. Nitrogen, without the carbon dioxide, could be safely returned to the atmosphere. Carbon dioxide, which is a greenhouse gas, could be stored instead of released to the atmosphere. Unfortunately, a problem with returning nitrogen to the atmosphere without the carbon dioxide is that it is difficult to separate nitrogen gas from carbon dioxide gas and typically requires large amounts of energy. There is also a problem with storing carbon dioxide gas, in that tanks having large volumes are required for handling, transportation, and storage.

As a ship consumes fuel during a voyage, the weight and balance changes to a point that the ship may become unstable. Formerly, ships added ballast in the form of seawater to fuel tanks to replace the consumed fuel. The seawater was discharged from the fuel tanks to take on new fuel when the ship reached port. Unfortunately, some fuel was inevitably discharged along with the seawater ballast. This pollution has become an unacceptable form of pollution. Currently, the practice of mixing seawater and fuel in tanks is largely outlawed in most jurisdictions. Separate tanks are now required for seawater ballast and for fuel. Unfortunately, the ballast tanks require a substantial volume within the ship that could otherwise be used for cargo. Moreover, the requirement of separate ballast and fuel tanks inevitably results in a shift in the weight and balance for the ship as fuel is consumed in a fuel tank at a first position on the ship while a ballast tank is filled with seawater at a second position on the ship. This increases the difficulty in designing a fuel/ballast system for achieving optimal weight and balance in all axes for various configurations of fuel consumption. Consequently, the total load and operating envelope of a ship are decreased.

SUMMARY

In an embodiment of the presently claimed invention, a liquefied natural gas (LNG) tank provides natural gas for burning in an engine. Exhaust emitted from the engine includes nitrogen gas, water vapor, and carbon dioxide. The exhaust gas may be cooled using heat exchange techniques, for example, reverse flow heat exchangers. Water may be separated from the exhaust gas using condensation techniques. Carbon dioxide may be separated from the remaining nitrogen. For example, a heat exchanger in combination with a turbo charger may be used to liquefy the carbon dioxide gas at high pressures and separate the liquid carbon dioxide from nitrogen gas. The liquid carbon dioxide may be sequestered or used industrially. Separated carbon dioxide gas may be further cooled to solid carbon dioxide and stored in the LNG tank, replacing the LNG burned in the engine. The solid carbon dioxide in the LNG tank may be used as ballast. Alternatively, the solid carbon dioxide may be sequestered using storage technology or used industrially.

In an embodiment of the presently claimed invention, a LNG system is provided for producing power and sequestering emitted carbon dioxide. The system comprises an engine configured to use natural gas for fuel and to emit an exhaust gas including carbon dioxide. The system also includes a LNG tank configured to store LNG and solid carbon dioxide. The system further comprises a first reverse flow heat exchanger in fluid communication with the engine. The first reverse flow heat exchanger is configured for exchanging heat between cold natural gas and carbon dioxide. The exchanged heat may be used to warm the cold natural gas for fuel for the engine and to cool the carbon dioxide to cold carbon dioxide. A second reverse flow heat exchanger may be further included in the system. The second reverse flow heat exchanger may be in fluid communication with the first reverse flow heat exchanger and LNG in the LNG tank. The second reverse flow heat exchanger is configured for exchanging heat between the LNG and the cold carbon dioxide. The exchange of heat may be used to provide heat to the LNG for a phase change from liquid natural gas to cold natural gas and to remove heat from the cold carbon dioxide. The removal of heat from the carbon dioxide provides for a phase change of the carbon dioxide from gas to solid. The solid carbon dioxide may be stored in the LNG tank. The system also includes a refrigerator configured to receive a portion of the cold natural gas from the LNG tank. The refrigerator may be used to condense the cold natural gas to LNG for storage in the LNG tank. The refrigerator may use energy from the engine for condensing the cold natural gas to LNG.

In an embodiment of the presently claimed invention, a method is provided for storing carbon dioxide emitted from a natural gas engine. In this method, natural gas may be received from a LNG tank and burned in the natural gas engine to produce exhaust gas which includes carbon dioxide and nitrogen. The method includes compressing and cooling the exhaust gas to a liquid carbon dioxide temperature. The method further includes separating the liquid carbon dioxide from the nitrogen and decompressing the separated carbon dioxide. The method also includes exchanging heat between the decompressed carbon dioxide and the LNG. The exchange of heat may be used to heat the LNG to cold natural gas and to cool the decompressed carbon dioxide to below solid carbon dioxide temperature. The solid carbon dioxide is stored in the LNG tank. The method also includes providing a first portion of the cold natural gas to the natural gas engine and condensing a second portion of the cold natural gas to LNG. The condensed liquefied natural LNG may be returned to the LNG tank for storage.

In an embodiment of the presently claimed invention, a LNG system is described for providing power and sequestering emitted carbon dioxide. The system includes an engine configured to use natural gas for fuel and a LNG tank configured to store LNG and solid carbon dioxide. The engine may emit an exhaust gas including carbon dioxide gas. The system further includes a first reverse flow heat exchanger in fluid communication with the engine. The first reverse flow heat exchanger may be configured for exchanging heat between cold natural gas and the emitted carbon dioxide gas. The exchange of heat may be used to warm the cold natural gas to be used for fuel for the engine and to cool the emitted carbon dioxide gas to cold carbon dioxide. The system also includes a separator in fluid communication with the first reverse flow heat exchanger. The separator may be configured to receive the exhaust gas including nitrogen and the cold carbon dioxide from the first reverse flow heat exchanger and to separate the cold carbon dioxide from the nitrogen. A second reverse flow heat exchanger is also provided. The second reverse flow heat exchanger is in fluid communication with the cold carbon dioxide from the separator and LNG in the LNG tank. The second reverse flow heat exchanger may be configured for exchanging heat between the LNG and the cold carbon dioxide. The heat exchange may be used to provide heat to the LNG for a phase change from LNG to cold natural gas. The heat exchange may also be used to remove heat from the cold carbon dioxide for a phase change from gas to solid carbon dioxide. The solid carbon dioxide may be stored in the LNG tank. The system further includes a refrigerator that is configured to receive a portion of the cold natural gas from the LNG tank. The refrigerator may condense the portion of cold natural gas to LNG and provide the LNG to the LNG tank.

In an embodiment of the presently claimed invention, a method is provided for storing ballast in a ship. The method includes removing LNG from a LNG tank and burning the removed LNG in an engine to produce energy for driving the ship. Exhaust gas emitted by the engine includes carbon dioxide gas and nitrogen gas. The method further includes separating the carbon dioxide gas from nitrogen gas in the exhaust gas using a compressor to compress the exhaust gas, a turbine to drive the compressor, and a heat exchanger. The method also includes cooling the carbon dioxide gas to produce solid carbon dioxide, and storing the solid carbon dioxide in the LNG tank. The solid carbon dioxide may serve as ballast in place of the removed LNG.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart illustrating temperatures for phase changes of various molecules.

FIG. 2 is a block diagram illustrating an exemplary embodiment of a LNG storage and natural gas engine system for sequestration of carbon dioxide, according to aspects of the invention.

FIG. 3 is a phase diagram illustrating energy and temperatures for the sequestration of carbon dioxide of FIG. 2.

FIG. 4 is a block diagram illustrating details of a nitrogen and carbon dioxide separator of FIG. 2.

FIG. 5 is a block diagram illustrating details of a LNG tank of FIG. 2.

FIG. 6 is an alternative embodiment of a system for sequestration of carbon dioxide.

FIG. 7 is a flow diagram of an exemplary process for sequestering carbon dioxide.

FIG. 8 is a flow diagram of an exemplary process for using carbon dioxide as ballast.

DETAILED DESCRIPTION

FIG. 1 is a chart 100 illustrating temperatures for phase changes of various molecules. The temperatures illustrated in FIG. 1 are approximate temperatures in degrees Celsius for phase changes that occur at about one atmosphere pressure. The illustrated phase changes include vapor to liquid for water (100°), gas to liquid for LNG (−163°), liquid to solid for LNG (−182°), and gas to solid (sublimation) for carbon dioxide to dry ice (−79°). The chart 100 further illustrates phase changes including gas to liquid for nitrogen dioxide (+21°), butane (0°), propane (−42°), ethane (−89°), nitric oxide (−152°), oxygen (−183°), carbon monoxide (−192°), and nitrogen (−196°). Additional phase changes illustrated in the chart 100 include liquid to solid for water (0°), nitrogen dioxide (−11°), and nitrous oxide (−164°). Natural gas includes primarily methane. However, natural gas includes ethane, propane, butane and other hydrocarbons. Since natural gas is primarily methane, the terms natural gas and methane and the terms LNG and liquid methane will be used interchangeably for simplicity.

FIG. 2 is a block diagram illustrating an exemplary embodiment of a LNG storage and natural gas engine system 200 for sequestration of carbon dioxide, according to aspects of the invention. The engine system 200 includes a LNG tank 210, separator 220, a heat exchanger 230, an environmental cooler 240, an optional regenerative heater 250, an engine 260, a refrigerator 270, and an optional air/nitrogen mixer 280. A heat exchange and direction of heat flow in the figures may be indicated using an arrow. The engine 260 burns fuel 262 using ambient air 266 and emits hot exhaust gas 264. The fuel 262 of FIG. 2 includes methane received from the LNG tank 210. However, other fuels may be burned including diesel, gasoline, fuel oil, and/or the like. It should be understood that when LNG is burned in an engine, the LNG is generally first heated to natural gas for burning in the engine. While LNG includes additional gases, methane is the largest component and the additional gasses will be ignored, for simplicity.

In the exemplary embodiment of FIG. 2, methane may be burned in the engine 260 using ambient air 266 according to the following chemical equation.

CH₄+2O₂+8N₂→CO₂+2H₂O+8N₂

According to the above chemical equation, for every 1 mole of methane (16 g CH₄), 2 moles of oxygen gas (32 g O₂) are consumed in a combustion reaction. Ambient air includes approximately 4 moles of nitrogen gas for each 1 mole of oxygen. Thus, 8 moles of nitrogen (224 g N₂) are mixed with the 2 moles of oxygen in ambient air 266 and are introduced together into the engine during combustion. The output of the above combustion reaction is hot exhaust gas 264 comprising a mixture of 1 mole of carbon dioxide (44 g CO₂), 2 moles of water vapor (36 g H₂O), and 8 moles of nitrogen gas. The 224 g of N₂ that was taken into the engine as part of the ambient air is expelled in the exhaust and mixed with the 44 g of carbon dioxide and 36 g of water vapor.

Thus, the components of the hot exhaust gas 264 include about 224 g nitrogen gas, 44 g carbon dioxide gas, and 36 g water vapor. Generally, additional byproducts are produced in the above combustion reaction. Examples include oxides of nitrogen (NOx), oxides of sulfur (SOx), hydrocarbons (HC), particulate matter (PM), and other emission gasses such as carbon monoxide (CO) and etc. For simplicity, however, these gasses may be ignored.

While FIG. 2 illustrates burning 1 mole of methane, other amounts of methane or fuel may be used. FIG. 2 is also illustrative of a flow of fuel. For example, a flow of 11 moles of intake gases per unit time may be burned in the engine 260 while emitting a flow of 11 moles of exhaust gas per unit time. The flow of intake gases may include 1 mole of methane, 2 moles of oxygen gas and 8 moles of nitrogen gas per unit time. The flow of exhaust gas may include 1 mole of carbon dioxide, 2 moles of water vapor, and 8 moles of nitrogen per unit time. The masses used as examples in this description for molecules such as methane, oxygen, nitrogen, water and other etc., are approximations based on molecular weights.

The regenerative heater 250 and the environmental cooler 240 may be used for cooling the hot exhaust gas 264 from engine exhaust temperatures to ambient temperature. Water may also be condensed out of the exhaust using the regenerative heater 250 and the environmental cooler 240. For simplicity, +300° C. may be treated as engine temperature or hot exhaust temperature. Also for simplicity, +40° C. may be treated as ambient temperature and +80° C. may be treated as warm temperature. However, ambient temperature may range from about 0° to +80° C. Engine temperature and engine exhaust temperature may range from +80° to +500° C. Warm exhaust, warm carbon dioxide, warm nitrogen, and/or warm fuel may be at a temperature between hot engine temperature and ambient temperature.

The regenerative heater 250 is configured to exchange heat between the engine temperature exhaust gas 264 (including carbon dioxide) and warm fuel 252, for cooling the hot exhaust gas 264 and further heating the warm fuel 252. The regenerative heater 250 may receive the engine temperature exhaust gas 264 from the engine 260 at about +300° C. and warm fuel 252 from the environmental cooler 240 at about +80° C. The heat exchange in the regenerative heater 250 may be used to cool the exhaust gas to about +80° C. for output to the environmental cooler 240 as warm exhaust gas 254. The heat exchange may be used to further heat the warm fuel 252 to about +300° C. for output to the engine 260 as hot fuel 262.

The environmental cooler 240 is configured to exchange heat between the warm exhaust gas 254 and the fuel 242. The environmental cooler 240 may also be used to condense water vapor from the warm exhaust gas 254. The environmental cooler 240 may receive the warm exhaust gas 254 at about +80° C. from the regenerative heater, and may receive the fuel 242 at about −45° C. from the heat exchanger 230. The heat exchange in the environmental cooler 240 may be used to condense most of the water vapor out of the warm exhaust gas 254 for output as water 246. The water 246 may be returned to the environment. The heat exchange in the environmental cooler 240 may be used further to cool the warm exhaust gas 254 from about +80° C. to about +40° C. for output to the heat exchanger 230 as ambient temperature exhaust gas 244. Since most of the water has been removed, the ambient temperature exhaust gas 244 output to the heat exchanger 230 includes about 224 g of nitrogen gas and 44 g of carbon dioxide gas. The heat exchange may be used to heat the fuel 242 from about −45° C. to about +80° C. for output to the regenerative heater 250 as fuel 252.

While cooling the hot exhaust gases has been described as being performed in two steps, more or fewer steps may be used for cooling the hot exhaust gases to about ambient temperature.

While fuel may be used for cooling the hot exhaust gas 264 from engine exhaust temperature to ambient temperature, other techniques that may be understood by persons having ordinary skill in the art may be used for cooling the exhaust and heating the fuel. For example, the environmental cooler 240 and/or the regenerative heater 250 may use cold water and/or air for cooling the exhaust gas 264 and/or warm exhaust gas 254 to ambient temperature.

The heat exchanger 230 is configured exchange heat between the ambient temperature exhaust gas 244 and cold fuel 232, and further to exchange heat between the ambient temperature exhaust gas 244 and cold nitrogen gas 236. The heat exchanger may receive ambient temperature exhaust gas 244 at about +40° C. from the environmental cooler 240, and cold fuel 232 at about −163° C. from the LNG tank 210. The heat exchanger may receive cold nitrogen gas 236 at approximately −70° C. from the separator 220. The heat from the ambient temperature exhaust gas 244 may be transferred to the cold nitrogen gas 236 and the fuel 232 to cool the ambient temperature exhaust gas 244 to about −70° C. The cooled exhaust gas may be output to the separator 220 as cold exhaust gas 234.

Approximately 36 joules of heat energy may be transferred to about 8 moles or 224 g of cold nitrogen gas 236 from the ambient temperature exhaust gas 244 in the heat exchanger 230. The transferred heat may be used to warm the about 8 moles of cold nitrogen gas 236 from about −70° C. to about +40° C. About 8 moles of warmed nitrogen gas may be returned to ambient air in the environment as nitrogen gas 238. A portion of the nitrogen gas 238 may be provided to the air/nitrogen mixer 280 for mixing with ambient air and/or oxygen. The air/nitrogen mixer 280 may provide ambient air 266 to the engine 260.

Approximately 4 joules of heat energy may be transferred from the ambient temperature exhaust gas 244 to about 1 mole of cold fuel 232 in the heat exchanger 230. The transferred heat may be used to warm the about 1 mole of cold fuel 232 from about −163° C. to about −45° C. The warmed fuel may be output as fuel 242 to the environmental cooler. The 36 joules and 4 joules of transferred heat are set forth for illustration purposes. The actual amount of heat energy may be more or less depending on input temperatures and rates of flow of the various gases and efficiency of the heat exchanger 230.

The separator 220 is configured to separate nitrogen from carbon dioxide. The separator 220 may use fractional distillation and heat exchange techniques, as described elsewhere here. The separator may receive cold exhaust gas 234 from the heat exchanger 230 at a temperature of about −70° C. The cold exhaust gas 234 includes about 1 mole of carbon dioxide and about 8 moles or 224 g of nitrogen. The separator 220 may output about 1 mole of cold nitrogen gas 236 to the heat exchanger 230 at about −70° C. The separator 220 may output about 1 mole of cold carbon dioxide gas 216 to the LNG tank 210 at about −70° C.

The LNG tank 210 is configured to store LNG 212 and solid carbon dioxide 218 or dry ice at a temperature of about −170° C. and a pressure of about 4-5 psi above 1 atmosphere (ATM). The LNG tank 210 may receive the 1 mole of cold carbon dioxide gas 216 from the separator 220 at about −70° C. The carbon dioxide gas 216 may be cooled from a gas at about −70° C. gas to a solid at about −170° C. (dry ice) for storing in the LNG tank. As discussed above, 1 mole of carbon dioxide has a mass of about 44 g. In cooling 44 g of carbon dioxide from −70° C. gas to −170° C. solid, about 30 joules of heat energy are released from the carbon dioxide.

The LNG 212 inside the LNG tank 210 may be used for some or all of the further cooling of the cold carbon dioxide gas 216 to solid carbon dioxide 218. For example, the 30 joules of heat energy released from the cold carbon dioxide gas 216 may be used to warm about 130 g of liquid methane from about −170° C. to about −163° C. and to cause a phase transition of LNG 212 to methane gas 214 at about −163° C. This can result in the conversion of about 130 g of LNG 212 to methane gas 214 inside the LNG tank. The 130 g of methane gas 214 may be released from the LNG tank 210. About 16 g of the methane gas 214 may be provided to the heat exchanger as the cold fuel 232. About 114 g of the methane gas 214 may be provided to the refrigerator 270 as recycle gas 272. The recycle gas 272 may be cooled to LNG at about −170° C. using the refrigerator 270 and output to the LNG tank 210 as liquid methane 274. Energy for operating the refrigerator 270 may be provided by the engine 260. The solid carbon dioxide 218 has a higher density than LNG 212 so the solid carbon dioxide 218 will tend to settle to the bottom of the LNG tank 210.

FIG. 3 is a phase diagram 300 illustrating energy and temperatures for the sequestration of carbon dioxide of FIG. 2. The horizontal axis of the phase diagram 300 represents energy in joules and the vertical axis represents temperature in degrees Celsius. The phase diagram 300 is not to scale. The temperatures and energies are approximate. The phase diagram 300 illustrates an ideal cycle for a gas mixture at +40° C. beginning at state 310. The gas mixture at state 310 includes carbon dioxide and nitrogen. An example of the carbon dioxide and nitrogen gas mixture at state 310 includes the ambient temperature exhaust gas 244. For simplicity, other gases that may be in the gas mixture will be ignored. During transition 312, the gas mixture undergoes a cooling from state 310 to state 314. At state 314, the gas mixture is at about −70° C. An example of the carbon dioxide and nitrogen gas includes the cold exhaust gas 234. The carbon dioxide and nitrogen gas mixture may be cooled, for example, using the heat exchanger 230.

At state 315, the carbon dioxide gas is separated from the nitrogen gas. The carbon dioxide gas and the nitrogen gas may be separated, for example, using the separator 220 to produce cold carbon dioxide gas 216 and cold nitrogen gas 236. The temperature of the separated carbon dioxide gas and nitrogen gas at state 315 is about −70° C.

During the transition 316 from state 315 to state 318, carbon dioxide gas undergoes a cooling from −70° C. to −79° C. A negligible amount of energy is release during the transition 316 for cooling 44 g of carbon dioxide. At state 318, the carbon dioxide gas is in equilibrium with solid carbon dioxide at about −79° C.

During the transition 320 from state 318 to state 322, the separated carbon dioxide undergoes a phase change from gas to a solid. The temperature does not change during the transition 320. Approximately 26 joules of heat energy are released during the transition 320 for the phase change. At state 322, the separated carbon dioxide is a solid. During the transition 324 from state 322 to state 326, solid carbon dioxide is cooled from −79° C. to −170° C. Approximately 5 joules of heat energy are released during the transition 324. At state 326, the carbon dioxide is a solid and methane is a liquid at about −170° C. Solid carbon dioxide may be stored in LNG at state 326. A portion or all of the transitions 316, 320, and 324 may take place inside the LNG tank 210.

During the transition 328 from state 326 to 330, about 130 g of methane is warmed from −170° C. to −163° C. At least 2 joules of energy are used to warm the methane −170° C. to −163° C. During the transition 332 from state 330 to state 334, the 130 g of methane undergoes a phase change from liquid to gas. The temperature does not change during the transition 332. At least 28 joules are required for the phase change of transition 332. The transitions 328 and 332 may take place inside the LNG tank 210. The approximately 30 joules of energy released during the transitions 316, 320 and 324 may be used to provide the 30 joules to perform the temperature change transition 328 and phase change transition 332. That is, energy released from the temperature change and phase change of 44 g of carbon dioxide may be absorbed in a temperature change and phase change of 130 g of methane.

At state 334, 114 g of the 130 g of methane may be refrigerated to liquid methane at −170° C., and returned to the LNG tank 210. This refrigeration step may be performed, for example, using the refrigerator 270. This refrigeration step is omitted from the phase diagram 300 for clarity. During the transition 336 from state 334 to state 338, 16 g of methane (the remainder of the 130 g of methane) are warmed from −163° C. to −70° C. At least 4 joules of energy are used to warm the 16 g of methane from −163° C. to −70° C. The transition 338 may be performed in the heat exchanger 230.

At state 315, 44 g of carbon dioxide gas was separated from 224 g of nitrogen gas. The transition of the nitrogen gas from the state 315 to state 338 is omitted for clarity. The 224 g of nitrogen gas and 16 g of methane may undergo at transition 340 from state 338 to state 342. The nitrogen gas may be maintained separate from the methane gas. Energy released in the transition 312 may be provided for the transition 340, for example, using the environmental cooler 240 and/or the heat exchanger 230. At state 342, the nitrogen gas may be released to the environment. While the state 338 and 342 may be at about the same position on the phase diagram as states 315 and 310 respectively, the states 338 and 342 are illustrated in FIG. 3 as being offset from the states 315 and 310 respectively for clarity.

FIG. 4 is a block diagram illustrating details of the nitrogen and carbon dioxide separator 220 of FIG. 2. Separator 220 includes a turbocharger 410, a separation heat exchanger 420, and a separation tank 430. The turbocharger 410 includes a first turbine 412, a compressor 414 and a second turbine 416. The compressor 414 is configured to receive the cold exhaust gas 234 from the heat exchanger 230. The cold exhaust gas includes carbon dioxide gas and nitrogen gas at about −70° C. and 1 ATM. The compressor is may compress the cold exhaust gas 234 to about 10 ATM at about +110° C. for output as hot compressed exhaust gas 424.

The separation heat exchanger 420 is configured exchange heat between the hot compressed exhaust gas 234 and cold separated nitrogen gas 436, and further to exchange heat between hot compressed exhaust gas 234 and liquid carbon dioxide 432. The heat exchanger may receive hot compressed exhaust gas 424 at about +110° C. from the compressor 414, and cold separated nitrogen gas 436 at about −40° C. from the separator tank 430. The heat exchanger may receive liquid carbon dioxide 432 at about −40° C. from the separator tank 430. The heat from the hot compressed exhaust gas 424 may be transferred to the cold separated nitrogen gas 436 and the liquid carbon dioxide 432 to cool the hot compressed exhaust gas 234 to about −40° C. for output as cold compressed exhaust gas 434. The cold compressed exhaust gas 434 includes liquid carbon dioxide and cold nitrogen gas. The cold compressed exhaust gas 434 may be provided to the separator tank 430.

The separator tank 430 is configured to allow liquid carbon dioxide to accumulate at the bottom and cold nitrogen gas to accumulate at the top. Cold separated nitrogen gas 436 may be removed from the top of the separator tank 430 and provided to the heat exchanger 420 for cooling the hot compressed exhaust gas 424. In cooling the hot compressed exhaust gas 424, the nitrogen gas is heated to hot compressed nitrogen gas 426 at about +110° C. and provided to the second turbine 416. The hot compressed nitrogen gas 426 may be used to drive the second turbine 416 and is output from the second turbine 416 as cold nitrogen gas 236 at about 1 ATM and −70° C. Similarly, the liquid carbon dioxide 432 may be removed from the bottom of the separator tank 430 and provided to the heat exchanger 420 for cooling the hot compressed exhaust gas 424. In cooling the hot compressed exhaust gas 424, the liquid carbon dioxide is heated to hot compressed carbon dioxide gas 422 at about +110° C. and provided to the first turbine 412. The hot compressed carbon dioxide gas 422 may be used to drive the first turbine 412 and is output from the turbine as cold carbon dioxide gas 216 at about 1 ATM. Power from the first turbine 412 and the second turbine 416 may be used to drive the compressor 414.

FIG. 5 is a block diagram illustrating details of the LNG tank 210 of FIG. 2. The LNG tank 210 is configured to receive cold carbon dioxide gas 216 from the separator 220 at about −70° C. The cold carbon dioxide gas 216 may be injected into LNG 212 in the LNG tank 210. The LNG 212 rapidly cools the injected carbon dioxide bubbles 540 to about −170° C. As the LNG 212 cools the carbon dioxide bubbles 540, the bubbles quickly condense to solid carbon dioxide flakes 532 having a consistency of “snow.” The solid carbon dioxide flakes 532 have a density of about 1.4-1.6 grams per cubic centimeters (g/cc) while LNG has a density of about 0.40.5 g/cc. Thus, the solid carbon dioxide flakes 532 settle to a layer of solid carbon dioxide 530 (“dry ice”) at the bottom of the LNG tank 210. The solid carbon dioxide 530 may be collected from the bottom of the LNG tank 210 using techniques understood by persons having ordinary skill in the art.

As discussed elsewhere herein, heat released in cooling the carbon dioxide from about −70° C. to about −170° C. turns LNG 212 into a gas, generating methane bubbles 520. The methane bubbles 520 collect at the top of the tank as cold methane gas 214. A portion of the methane gas 214 may be provided as fuel 232 to the engine 260, via the heat exchanger 230. Another portion of the methane gas 214 may be refrigerated a using the refrigerator 270 and returned to the LNG tank 210 as liquid methane 274.

Solid carbon dioxide 218 may be stored in the LNG tank 210 as ballast to replace LNG 212 that is consumed as fuel in the engine 260. As discussed elsewhere herein, about 44 g of carbon dioxide are produced for each 16 g of methane consumed. The 16 g of LNG 212 may be replaced in the LNG tank 210 by the 44 g of solid carbon dioxide. The volume of 16 g of LNG 212 is about 32-40 cc. The volume of 44 g of solid carbon dioxide 218 is about 28-31. Thus, volume of the LNG 212 plus solid carbon dioxide stored in the LNG tank 210 remains approximately constant or decreases as LNG 212 is used for fuel. However, the weight of the ballast in the form of solid carbon dioxide 218 at the bottom of the LNG tank 210 increases at a ratio of about 2.75:1 dry ice: LNG respectively (i.e., 44 g ballast: 16 g fuel). The increased weight of the ballast replacing the consumed fuel enhances stabilization of the ship. Moreover, the solid carbon dioxide 218 forms ballast at the bottom of the LNG tank 210 which is an advantageous location for stabilizing a ship. Using solid carbon dioxide 218 as ballast in the LNG tank 210 to replace the consumed fuel reduces or eliminates the need for separate tanks to hold seawater ballast to replace the consumed fuel.

Various examples of temperatures, pressures, energy transfers and amounts of fuel are provided in this description for methane, carbon dioxide, nitrogen, and water. These examples are for illustrating the processing of carbon dioxide from engine exhaust to dry ice and sequestration of the dry ice inside an LNG tank. However, these temperatures, pressures, energy transfers and amounts should not be considered in a limiting sense. Other temperatures, pressures, energy transfers and amounts may be used without departing from the invention.

The solid carbon dioxide may be removed from the LNG tank 210 and sequestered or used industrially. For example, the solid carbon dioxide may be stored deep under layers of rock. Solid carbon dioxide may be stored at the bottom the ocean. For example, carbon dioxide may be injected into deep-sea formations such as underwater basalt. Below about 500-1,000 feet, carbon dioxide is liquid at seawater temperatures. Thus, solid carbon dioxide would warm to liquid carbon dioxide in the deep sea. Liquid carbon dioxide is denser than seawater and will sink to the bottom of the ocean.

The solid carbon dioxide may be used industrially. Dry ice may be used as a cyclic or non-cyclic refrigerant. For example, dry ice is used in the food industry for transport and storage of foods such as ice cream. Carbon dioxide may be used as a refrigerant in vehicles, freezers and refrigerators. Dry ice can also be used for construction, cleaning, plumbing, insect abatement, scientific experiments.

Solid carbon dioxide may be used as storage for later conversion to carbon dioxide gas. The carbon dioxide gas may be stored in stable carbonate mineral forms using a process referred to as mineral carbonation or mineral sequestration. Limestone is an example of a mineral carbonate. A substantial percentage of the mineral mass in the Earth's crust is able to form stable carbonates with carbon dioxide.

The carbon dioxide gas may be injected into the earth adjacent oil and natural gas wells for enhancing recovery of fuels such as methane, oil and natural gas. The injected carbon dioxide may reduce the viscosity of crude oil adjacent the well which can enhance a flow of oil through the earth toward the well. The injected carbon dioxide may also increase pressure in a well for removing the flow of oil and natural gas. For example, carbon dioxide may be used for enhancing recovery of methane from coal beds. A power plant may be operated on fuel received from a nearby oil or natural gas field. The carbon dioxide produced while generating power may be separated from the exhaust gas. The separated carbon dioxide may be injected into the field for sequestration and enhancing recovery of the fuel. Other emission gasses may be similarly separated. Thus, the power plant may be operated at near zero emissions.

FIG. 6 is an alternative embodiment of a system 600 for sequestration of carbon dioxide. FIG. 6 differs from FIG. 2 in that a liquid oxygen (LOX) tank 610 provides LOX 612 at about −183° C. to a refrigerator 670 for cooling the recycle gas 272. The LOX 612 may sink about 15 joules of heat energy from the recycle gas. The refrigerator 670 may use energy (e.g., from the engine 260) for removing about 11 joules from the recycle gas 272 to produce the LNG 274 for returning to the LNG tank 210. The refrigerator 670 may provide oxygen gas 614 to the mixer 280 for mixing with nitrogen gas. Alternatively, the refrigerator 270 may provide unmixed oxygen gas 614 to the engine 260.

For simplicity and clarity, carbon dioxide separation and sequestration discussions omitted emissions of other components which are typically present in exhaust gas at concentrations of about parts per million. However, these components, such as NOx, SOx, and various hydrocarbons, may be separated and sequestered in a similar manner. For example, NO₂ undergoes a phase transition from gas to liquid at about +21° C., butane at about 0° C., propane at about −42° C., ethane at about −89° C., and nitric oxide at about −152° C. These molecules may be separated and stored in liquid form using heat exchange in combination with turbo charger techniques as discussed elsewhere herein. Further, NO₂ undergoes a phase transition from liquid to solid at about −21° C. and SO₂ at about −75° C. NO₂ and/or SO₂ may be separated from other exhaust gases, cooled to a solid form, and stored as solids inside a LNG tank a manner similar to that described for carbon dioxide elsewhere herein.

FIG. 7 is a flow diagram of an exemplary process 700 for sequestering carbon dioxide, according to various embodiments of the technology. In step 702, natural gas from an LNG tank is burned in an engine. Exhaust gas including carbon dioxide and nitrogen is emitted from the engine. In step 704, the exhaust gas emitted from the engine is compressed. In step 706, the compressed exhaust gas from the compressor is cooled to temperature that results in the compressed carbon dioxide gas becoming a liquid while the compressed nitrogen remains a gas. At the same time, cold compressed nitrogen gas and liquid carbon dioxide may be warmed to hot compressed nitrogen gas and hot compressed carbon dioxide in a heat exchange with the compressed exhaust gas. In step 708, the liquid carbon dioxide and the cooled compressed nitrogen gas are separated. In step 710, the hot compressed nitrogen gas is decompressed using a first turbine. In step 712, hot compressed carbon dioxide is decompressed using a second turbine. The hot compressed carbon dioxide becomes cold carbon dioxide upon decompression. In step 714, heat is exchanged between cold carbon dioxide and LNG, for example, inside a LNG tank. Cold carbon dioxide becomes solid carbon dioxide as a result of the heat exchange while LNG becomes a methane gas. In step 716, solid carbon dioxide is stored, for example, inside the LNG tank. In step 718, a portion of the methane is provided to the engine for burning as a fuel. In step 720, a second portion of the methane is condensed to LNG. In step 722, the condensed LNG is stored in the LNG tank.

FIG. 8 is a flow diagram of an exemplary process 800 for using carbon dioxide as ballast. In step 802, LNG is removed from a LNG tank. In step 804, the removed LNG is burned in an engine. Heat may be used to convert the removed LNG to natural gas for burning in the engine. Exhaust including carbon dioxide and nitrogen is emitted from the engine. In step 806, carbon dioxide and nitrogen in the exhaust are separated. In step 808, the separated carbon dioxide gas is cooled to solid carbon dioxide. In step 810, the solid carbon dioxide is stored in the LNG tank in place of the removed LNG. The stored solid carbon dioxide may be used as ballast inside the LNG tank.

Several embodiments are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations are covered by the above teachings and within the scope of the appended claims without departing from the spirit and intended scope thereof. For example, LNG engine systems 200 and 600 are described for a ship. However, these systems may be used for other applications, including power plants, land vehicles, aircraft, and etc. For example, the engine 260 may use fuels other than LNG. Various embodiments of the invention include logic stored on computer readable media, the logic configured to perform methods of the invention.

In the foregoing specification, the present invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the present invention is not limited thereto. Various features and aspects of the above-described present invention may be used individually or jointly. Features in each of the various illustrations may be used individually or combined with features in other illustrations for illustrating the present invention. Further, the present invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art. 

1. A liquefied natural gas system for providing power and sequestering emitted carbon dioxide, the system comprising: an engine configured to use natural gas for fuel and emit carbon dioxide gas in an exhaust gas; a liquefied natural gas tank configured to store liquefied natural gas and solid carbon dioxide; a first reverse flow heat exchanger in fluid communication with the engine, the first reverse flow heat exchanger configured for exchanging heat between cold natural gas and the emitted carbon dioxide gas to warm the cold natural gas for fuel for the engine and to cool the emitted carbon dioxide gas to cold carbon dioxide; a second reverse flow heat exchanger in fluid communication with the cold carbon dioxide in the first reverse flow heat exchanger and liquefied natural gas in the liquefied natural gas tank, the second reverse flow heat exchanger configured for exchanging heat between the liquefied natural gas and the cold carbon dioxide to provide heat to the liquefied natural gas for a phase change from liquid natural gas to cold natural gas, and to remove heat from the cold carbon dioxide for a phase change from gas to solid carbon dioxide for storage in the liquefied natural gas tank; and a refrigerator configured to receive a portion of the cold natural gas from the liquefied natural gas tank, to condense the cold natural gas to liquefied natural gas, and to provide the liquefied natural gas to the liquefied natural gas tank.
 2. The system of claim 1, wherein the second reverse flow heat exchanger is disposed in the liquefied natural gas tank.
 3. The system of claim 1, wherein the refrigerator is further configured to receive energy from the engine and use the received energy to condense the portion of the cold natural gas to liquefied natural gas temperature.
 4. The system of claim 1, further comprising a separator in fluid communication with the first reverse flow heat exchanger and the second reverse flow heat exchanger, the separator configured to receive the exhaust gas including nitrogen and the cold carbon dioxide from the first reverse flow heat exchanger, to separate the cold carbon dioxide from the nitrogen and to communicate the cold carbon dioxide to the second reverse flow heat exchanger.
 5. The system of claim 4, further comprising a reverse flow environmental heat exchanger in fluid communication with the first reverse flow heat exchanger, the environmental heat exchanger configured for exchanging heat between ambient temperature natural gas and warm carbon dioxide to heat the ambient temperature natural gas to warm natural gas and to cool the warm carbon dioxide to about ambient temperature.
 6. The system of claim 5, further comprising a reverse flow regenerative heat exchanger in fluid communication with the environmental heat exchanger and the engine, the regenerative heat exchanger configured for exchanging heat between warm natural gas and hot carbon dioxide to heat the warm natural gas to about engine temperature and to cool the hot carbon dioxide to warm carbon dioxide.
 7. A method for storing carbon dioxide exhausted from a natural gas engine, the method comprising: burning natural gas received from a liquefied natural gas tank in the natural gas engine to produce exhaust gas including carbon dioxide and nitrogen; compressing the exhaust gas; cooling the compressed exhaust gas to a liquid carbon dioxide temperature; separating the liquid carbon dioxide and the nitrogen; decompressing the separated carbon dioxide; exchanging heat between the decompressed carbon dioxide and the liquefied natural gas to heat the liquefied natural gas to cold natural gas and to cool the decompressed carbon dioxide to below solid carbon dioxide temperature; storing the solid carbon dioxide in the liquefied natural gas tank; providing a first portion of the cold natural gas to the natural gas engine; condensing a second portion of the cold natural gas to liquefied natural gas; and storing liquefied natural gas in the liquefied natural gas tank.
 8. The method of claim 7, wherein compressing the exhaust gas is performed using a compressor and decompressing the separated nitrogen is performed using a first turbine configured to drive the compressor.
 9. The method of claim 8, further comprising decompressing the separated carbon dioxide using a second turbine configured to drive the compressor.
 10. The method of claim 7, wherein cooling the second portion of the cold natural gas to liquefied natural gas temperature comprises receiving energy from the engine and using the received energy for cooling the portion of the cold natural gas.
 11. The method of claim 7, wherein cooling the second portion of the cold natural gas to liquefied natural gas temperature comprises exchanging heat between the portion of the cold natural gas and liquid oxygen.
 12. The method of claim 7, wherein exchanging heat between the liquefied natural gas and the cold carbon dioxide is performed inside the liquefied natural gas tank.
 13. A system for storing solid carbon dioxide, the system comprising: a tank configured to store liquefied natural gas, the tank in fluid communication with exhaust gas including carbon dioxide gas emitted from a natural gas engine; a heat exchanger disposed in the tank and in fluid communication with the carbon dioxide gas, the heat exchanger configured for removing heat from the carbon dioxide gas and condensing the carbon dioxide gas to solid carbon dioxide, the heat exchanger further configured for providing the removed heat to the liquefied natural gas in the tank to convert a portion of the liquefied natural gas to natural gas, the tank configured for storing the solid carbon dioxide; a manifold configured to communicate a first portion of the natural gas from the tank to the natural gas engine; and a refrigerator in fluid communication with the tank, the refrigerator configured to receive a second portion of the natural gas from the tank and to condense the second portion of the natural gas to liquefied natural gas, the tank further configured to receive the condensed liquefied natural gas from the refrigerator.
 14. The system of claim 13, wherein the heat exchanger comprises a nozzle configured to introduce the carbon dioxide gas into the liquefied natural gas in the tank.
 15. The system of claim 13, wherein the refrigerator comprises liquid oxygen and a heat exchanger configured to exchange heat between the cold natural gas and the liquid oxygen.
 16. The system of claim 13, further comprising a liquid oxygen heat exchanger configured to exchange heat between carbon dioxide gas and liquid oxygen.
 17. A method for storing ballast in a ship, the method comprising: removing liquefied natural gas from a liquefied natural gas tank; burning the removed liquefied natural gas in an engine to produce energy for driving the ship and to produce exhaust gas including carbon dioxide gas; cooling the carbon dioxide gas to produce solid carbon dioxide; and storing the solid carbon dioxide as ballast in place of the removed liquefied natural gas in the liquefied natural gas tank.
 18. The method of claim 17, further comprising separating the carbon dioxide gas from nitrogen gas in the exhaust gas using a compressor, a turbine to drive the compressor, and a heat exchanger.
 19. The method of claim 17, further comprising: compressing the exhaust gas; cooling the compressed exhaust gas to liquid carbon dioxide temperature; separating the liquid carbon dioxide from nitrogen gas in the exhaust gas; exchanging heat between the compressed exhaust gas and the separated liquid carbon dioxide to cool the compressed exhaust gas; exchanging heat between the compressed exhaust gas and the separated nitrogen gas to cool the compressed exhaust gas; and driving a turbo charger using the separated nitrogen and carbon dioxide, the turbo charger configured for compressing the exhaust gas.
 20. The method of claim 19, further comprising sequestering a portion of the solid carbon dioxide in the ocean at a depth of at least 500 feet below the surface. 